U.S. patent number 5,138,424 [Application Number 07/606,460] was granted by the patent office on 1992-08-11 for positive working polyamic acid/imide photoresist compositions and their use as dielectrics.
This patent grant is currently assigned to Brewer Science, Inc.. Invention is credited to Terry Brewer, Ruth M. Cuzmar, Tony D. Flaim, Dan W. Hawley, Mary G. Moss.
United States Patent |
5,138,424 |
Moss , et al. |
August 11, 1992 |
Positive working polyamic acid/imide photoresist compositions and
their use as dielectrics
Abstract
Positive working polyamic acid photoresist compositions are
disclosed having improved high resolution upon image development
and exhibiting stable photosensitivity and superior dielectric
performance. The compositions comprise polyamic acid condensation
products of an aromatic dianhydride and an aromatic di-primary
amine wherein a percentage of the diamine comprises special
dissolution inhibiting monomers. The compositions may be further
improved by the presence of particular supplemental additives.
Inventors: |
Moss; Mary G. (Rolla, MO),
Brewer; Terry (Rolla, MO), Cuzmar; Ruth M. (Rolla,
MO), Hawley; Dan W. (St. James, MO), Flaim; Tony D.
(St. James, MO) |
Assignee: |
Brewer Science, Inc. (Rolla,
MO)
|
Family
ID: |
26952831 |
Appl.
No.: |
07/606,460 |
Filed: |
October 31, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
268023 |
Nov 7, 1988 |
5024922 |
|
|
|
Current U.S.
Class: |
257/759;
257/E23.167; 257/E21.259 |
Current CPC
Class: |
G03F
7/039 (20130101); H01L 21/312 (20130101); H01L
23/5329 (20130101); H01L 2924/0002 (20130101); H01L
21/02304 (20130101); H01L 21/02282 (20130101); H01L
21/02118 (20130101); H01L 21/02348 (20130101); H01L
2924/0002 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
H01L
21/02 (20060101); H01L 23/52 (20060101); H01L
21/312 (20060101); G03F 7/039 (20060101); H01L
23/532 (20060101); H01L 029/34 (); H01L
023/28 () |
Field of
Search: |
;357/52,54,72 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Carroll; J.
Attorney, Agent or Firm: Peoples, Jr.; Veo
Parent Case Text
This is a divisional of copending application(s) Ser. No.
07/268,023 filed on Nov. 7, 1988 now U.S. Pat. No. 5,024,922.
Claims
What is claimed:
1. A multilevel metal or oxide article of manufacture on an
integrated circuit having a photoimaged dielectric insulation
between the metal or oxide levels said dielectric insulation
comprising:
(a) the condensation product of an aromatic dianhydride and an
aromatic di-primary amine having from about 10 to about 50 mole
percent of the di-primary amine having the formula: ##STR2## where
R contains one or more phenyl rings connected by oxygen, SO.sub.2,
alkyl, fluoroalkyl, or biphenyl linkages; and
(b) a diazoquinone photoactive sensitizer;
(c) from 10 to 20 solids weight per cent of a second polymer to
improve developing latitude, said second polymer selected from the
group consisting of novolac resin, dimethylamino-benzaldehyde,
diisopropyl amine, trihexyl amine, dibenzyl amine,
N-(triethoxysilylpropyl) urea, p-dimethylamino benzaldehyde,
isocyanatopropyl- triethoxysilane and mixtures thereof;
(d) from 1% to about 12% by weight of a compound selected from the
group consisting of polyurethanes and cyclo aliphatic
diepoxides;
(e) the dielectric insulation is characterized by less tendency to
outgas to form pinholes, greater solubility difference between
exposed and unexposed regions, unexposed and undeveloped regions
having resistivity in the range of 1.times.10.sup.16 ohm cm., a
dielectric strength greater than 5.times.10.sup.5 volt/cm, while
also having less susceptibility to cracking due to mismatch of
thermal coefficients and the insulation layer serving as a
photoresist which need not be removed after imaging.
2. The article of claim 1 wherein the diepoxide is 3, 4
epoxycyclohexylmethyl -3', 4'- epoxycyclohexane carboxylate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to positive working polyamic acid
photoresist compositions and their use as dielectrics, and
particularly to improvements in imagewise resolution upon photo
image development.
2. Description of the Prior Art
Photoresist compositions, generally, are well-known in the art as
coatings comprising a diazoquinone photo sensitizer and a resinous
binder. These compositions are coated or deposited onto certain
substrates and when exposed to light or proper wave length
(irradiated) the compositions are chemically altered in their
solubility to certain solvents (developers). This process is known
as photoimage development.
Two types of photoresist compositions are known, namely; negative
working and positive working photoresist. The negative working
resists are compositions which are initially soluble in the
developers, but following irradiation become insoluble.
Accordingly, by configuring a specific pattern of irradiation,
during photoimage development, those areas of the coating exposed
to the light will form raised lines which define negative images.
Positive working photoresist compositions work in the opposite
fashion. That is, the compositions are initially insoluble in
alkaline developer and accordingly, upon irradiation the exposed
regions dissolve and form indented lines or cavities that define
positive images.
The clarity and precision with which these liens are formed, called
resolution, is calibrated in microns of geometry.
In the microelectronics industry, it is important to achieve line
resolutions as small as possible, preferably one micron or smaller
for coating thicknesses within the range of one to two microns. It
is also desirable to employ positive working photoresist rather
than negative working photoresist in dielectric applications. The
recessed lines formed in the coating from the positive resist will
better serve dielectric applications than will raised lines formed
from negative resist because in dielectric applications, unlike
many other applications, the bulk of the coating remains on the
wafer during subsequent processing. Additionally, modern techniques
for processing semi-conductors call for plasma and sputter etching,
ion beam implantation, and the like, which require photoresist
compositions having stability at temperatures as high as
300.degree. and higher. When photoresist compositions are employed
as dielectric layers, not only is thermal stability essential, but
the resist must also maintain good dielectric properties.
Over the years, polyamic acid condensation resins produced from an
aromatic dianhydride and an aromatic di-primary amine, such as
those described in U.S. Pat. No. 3,179,634, have received
widespread attention as resinous binders for photoresist
compositions because they are readily converted by heat to
thermally stable polyimides. They are resistant to dilute acids and
organic solvents and they are heat stable at temperatures in excess
of 400.degree. C. However, because of the high solubility of the
polyamic acid in alkaline-developer, prior-to conversion to the
imide, their use has been restricted for the most part to negative
working photoresist compositions.
The earliest of these negative working polyamic acid photoresist
was disclosed in U.S. Pat. No. 3,623,870. Therein, a negative
working photoresist composition was disclosed comprising a mixture
of photosensitive dichromate and the polyamic acid binder. The
composition was initially soluble in the developer and upon
photoimaging, the dichromate cross-linked the polyamic acid binder
causing the exposed areas to become less soluble than the
uncross-linked polyamic acid of the unexposed areas. Thus,
development was said to proceed via different rates of solubility
between imagewise exposed and non-exposed areas. It was found,
however, to be difficult, if at all possible, to prevent some
attack by the developer solvent on the non-exposed areas;
accordingly, the problems associated with obtaining good resolution
from polyamic acid systems arose even in negative working polyamic
acid photoresist compositions.
More recently, in U.S. Pat. No. 4,451,551, polyamic acids
photosensitized with a compound having an amino group and an
aromatic azide group were disclosed in a negative working
photoresist composition. After photoimage development, the
composition was baked at from 150.degree. to 300.degree. C. to
produce a heat resistant polyimide that resisted distortions when
heated to 400.degree. C. for an hour and was used as dry-etching
resistant photoresist. Again, the composition was initially soluble
in the developer and the unexposed portions remained soluble, while
the irradiated portions were rendered relatively less soluble for
the formation of negative patterns.
More recently, in U.S. Pat. No. 4,515,887, a negative working
polyamic acid photoresist specifically designed for use as a
dielectric was disclosed. Therein, the base resin was produced by a
condensation reaction between aromatic dianhydride and a mixture of
aromatic diamine plus amine organo terminated polydiorganosiloxane.
The resultant silicone-polyamic acid was modified by a mixture of
isocyanato organoacrylate which enabled the polymer to be
sensitized with an appropriate photo sensitizer such as Michler's
ketone or benzophenone. The composition was then spin coated onto
the substrate and heated to 100.degree. C. for partial imidization.
Upon exposure to ultra-violet light in alkaline developer solution
cross-linking occurred capable of insolubilizing the exposed areas
and creating a negative patterned dielectric layer on its
surface.
Even more recently in U.S. Pat. No. 4,656,116, a negative working
polyamic acid photoresist composition was disclosed wherein
aromatic tetracarboxylic acid derivatives and aromatic diamines
having both ortho-positions relative to the phenylene radical
bonded to an imide group of the polymer and substituted by alkyl
groups where radiation-crosslinked with organic chromophoric
polyazides. The compositions were useful in preparing dielectric
layers and for producing printed circuits and integrated
circuits.
Several problems associated with the use of negative working
polyamic acid photoresists for imagable dielectric layers could be
overcome if positive working photoresists were used. In the first
place, dielectric applications commonly require that holes be
patterned in the existing coating. Hole patterning is most
effectively accomplished through the use of a positive resist, in
which the exposed areas are removed In addition, a slight side
slope is desired in order to achieve effective metal contacts.
Positive resists naturally achieve the necessary sloping sidewalls
because the top of the film receives a higher exposure than the
bottom of the film, and is therefore slightly more soluble. The
slope produced in a negative resist is in the opposite direction
from that desired, because solubility is inversely related to
exposure. A third advantage of positive resist is especially
important in thick films where absorption of the film can greatly
decrease sensitivity. Positive resists photobleach--that is, the
absorption coefficient decreases upon exposure, enabling more light
to reach the lower regions of the film, and increasing exposure
efficiency. For these reasons, a positive resist is preferable to a
negative resist for dielectric applications.
The earliest attempts to make thermally stable polyamic acid
photoresist compositions in a positive working fashion was
disclosed in U.S. Pat. No. 4,093,461. Therein, it was attempted to
make the polyamic acid condensation resin insoluble in an alkaline
developer by admixture with orthoquinone and orthonaphthoquinone
diazide photosensitizers. It was believed that sufficient
quantities of the diazide sensitizer would render the unexposed
areas of the photoresist composition completely insoluble in the
aqueous alkaline developing solution because of the hydrophobicity
and insolubility of the diazides themselves before photolysis or
photoimage development. Hence, a positive image could be formed on
the support corresponding to the master pattern of configurated
irradiation used during the photoimage development. It was believed
that a difference in solvation was required to create the positive
working phenomenon and/or complete insolubility in the developer as
by use of large quantities of the photosensitizer particularly the
abietyl types of diazides.
By use of this different solvation, it was believed that sharp
distinctions between imagewise exposed and non-exposed areas during
development would occur and thus ensure that only light-struck
areas were dissolved in the developer., whereas non-exposed areas
would remain insoluble and unaffected in the developer. However,
such attempts have had only limited success in that polyimide-based
photoresist systems exhibit such a high dissolution rate in
alkaline solutions with conventional, commercially available
diazide sensitizers, that adequate control over processes to obtain
high resolution is not possible.
Attempts to decrease the dissolution rate under the teachings of
U.S. Pat. No. 4,093,461, by increasing the concentration of the
sensitizer in the photoresist to as high as 50% by weight have been
problematic for two reasons: (1) The concomitant increase in
optical density of the photoresist inhibits full penetration of the
film thickness by the radiation source, and (2) there is a
progressive reduction in thermal stability of the photoresist
associated with increased concentration of the sensitizer.
More recently, positive working polyamic photoresist compositions
have been designed to overcome the problems associated with
increased sensitizer concentration.
In European Patent Application No. 224,680, it is disclosed that by
reducing the acidity of the polyamic acid by about 10 to about 40%
of its original value, the dissolution rate of the initial
unexposed photoresist composition in the alkaline developer and the
subsequent dissolution rate of the exposed areas could be
controlled to provide a tailored development rate within a desired
range. This reduction in acidity is achieved by, for example,
pre-baking to achieve partial imidization., or partial
neutralization with basic organic reagents including the use of
blends of the polyamic acid and its ester derivatives or
co-polymers of the acid and esters derived from the organic
reagents. However, pre-baking to achieve partial imidization is
therein taught to be limited in the extent to which it could be
employed to reduce acidity because temperatures above about
100.degree. C. as for example, even 120.degree. C., caused loss of
photosensitivity by degradation of the diazoquinone
photosensitizers. Furthermore, acidity reduction through employment
of the basic organic reagents, therein disclosed, tends to corrode
the conductors found in integrated circuits. Small mobile
impurities, created therefrom, tend to degrade dielectric
performance.
Accordingly, in spite of the fact that the microelectronics
industry's manufacturing processes are based almost exclusively on
positive photoresist techniques, those more desirable thermally
stable polyamic acid photoresists which are of any practical
utility continue to be negative working. The most prevalent
commercially available positive photoresist compositions continue
to consist of base resins made from phenol-formaldehyde
condensation products such as Novolacs. These Novolac based
photoresists, when mixed with standard photosensitizers, become
insoluble and thereafter allow exposed areas during irradiation or
photodevelopment to become soluble. Novolac melts at temperatures
above about 150.degree. C. with the result that such systems are
not suitable for the modern technology high temperature
applications, particularly as interlayer dielectric coatings for
integrated circuits.
It would therefore be a substantial advancement in the art to
develop a positive working polyamic acid photoresist composition
which was devoid of the above-described drawbacks in the prior
art.
SUMMARY OF THE INVENTION
It is a principal object of the present invention to provide a
novel positive working polyamic acid photoresist composition
capable of pre-baking at above 120.degree. C. prior to photoimage
development without loss of its photosensitivity or resolution.
It is a further principal object of the present invention to
provide a novel positive working polyamic acid photoresist without
the basic organic reagents which give rise to degradation of
dielectric performance.
It is a still further object of the present invention to provide a
positive working polyamic acid photoresist composition which
exhibits high resolution at less than two micron geometries.
It is an additional object of the present invention to provide
dielectric insulating layers, photoimagable at high resolutions and
of high thermal stability from positive working polyamic acid
photoresist where removal of the resist prior to baking is
negated.
These objects and others which will become apparent from the
following detailed description and examples of the invention are
made possible by a polyamic acid composition comprising at least
10% of particular diamine monomers and by the addition of
particular supplemental additives.
BRIEF DESCRIPTION OF THE DRAWINGS
The inventive and distinctive features of the invention are set
forth in the claims. The invention itself, however, taken with
further objects and advantages thereof may best be understood by
reference to the following description and the accompanying
drawings, in which:
FIG. 1 is an illustration of diamines for use in polyimide
photoresist formulations,
FIG. 2 is an illustration of dianhydrides for use in polyimide
photoresist formulations,
FIG. 3 is an illustration of positive polyimide layer, 11, for use
as a multilevel isolation between multilevel metal layers, 10,
and
FIG. 4 is an illustration of positive polyimide layers, 2, and
silicon dioxide layer, 3, for use as a multilevel isolation between
multilevel metal layers, 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Polyamic acid solubility in basic developer is very dependent upon
the presence of excess acidic or basic groups, and as a result,
polyamic acid is especially well suited to the production of a
positive photoresist. In forming a polyimide-based resist based on
diazonaphthoquinone sensitizer chemistry, the choice of the base
polyimide chemistry is critical. Most commonly, polyamic acid
solubility is regulated by thermally converting some of the
carboxylic acid groups to imides, which are not base-soluble. The
percentage of imidization is a function of the time of heating and
the temperature. For many common polyamic acids, such as the
polyimide precursor made from pyromellitic dianhydride and
4,4'-oxydianiline, temperatures in the range of
160.degree.-170.degree. C. are required to achieve the desired etch
rate in aqueous basic developer.
Because they contain highly reactive diazo groups, sensitizers
based on diazonaphoquinone are degraded at high temperatures. The
degradation products are not photosensitive, and in fact can be
detrimental to the lithographic properties because of the formation
of insoluble film due to crosslinking. Degradation of the
sensitizer follows first-order kinetics. The half-life of
decomposition at 130.degree. C. is eight minutes. By 150.degree.
C., the half-life decreases to 90 seconds. Proper removal of
solvent requires a hotplate bake of at least 90 seconds. Thus, a
low resist bake temperature is desirable to achieve the optimum
sensitivity and resolution without scumming.
The results on sensitizer degradation demonstrate that unmodified
PMDA-ODA polyamic acid cannot be used in a polyimide resist using
diazonaphthoquinone sensitizers because regulation of the
solubility by baking requires temperatures that destroy the
sensitizer. This fact has been previously stated in EPO patent
application 224 680. The mixing of a high percentage of sensitizer
into the polyamic acid gives the polymer the required solubility.
However, at these concentrations, the absorbance is so great that
the bottom of the film remains unexposed while the tope of the film
is overexposed. The applicants' invention consists of polyamic acid
resist formulations in which the solubility of the polyamic acid is
reduced in one or more of the following ways: by incorporating into
the polyimide structure one of a class of specific diamine monomers
of a type that reduce the intrinsic solubility of the polymer, by
the addition of a second polymer which itself has low solubility in
developer, or by the addition of a cyclic aliphatic diepoxide.
A generalized structure of the base polyamic acid is as
follows:
[(diamine 1)x- (dianhydride)- (diamine 2)y-(dianhydride)]n where
diamine 1 (has the formula: ##STR1## where R contains one or more
phenyl rings connected by oxygen, SO.sub.2, alkyl, fluoroalkyl, or
biphenyl linkages, diamine 2 is selected from the group of diamines
shown in FIG. 1, the dianhydride is selected from the group of
dianhydrides shown in FIG. 2, and the mole ratio of x to y ranges
from 100% to 5%, preferably 55% to 20% depending upon the desired
solubility and desired raw material source. The bake temperature
required to achieve low solubility can be controlled by adjusting
the ratio of diamine 1 to diamine 2.
Thus, a polyamic acid in which a percentage of the diamine is as
above-described, preferably either bis-4(4-aminophenoxy) phenyl
sulfone (BAPS) or bis-4(4-aminophenoxy) phenyl propane (BAP) has
the required solubility in developer following a prebake of from
about 110.degree. C. to 150.degree. C. but preferably 135.degree.
C. bake temperature. It is expected that the high aromaticity of
the diamine provides the means by which solubility is reduced. In
order to regulate the developing rate, a mixture of diamines is
used in which one diamine is BAPS or BAPP and the other diamine is
of the general structure shown in FIG. 1. For polymers made with
BAPP, approximately 10-20 mole % of the total diamine produces a
polymer with the proper solubility; for BAPS, approximately 45 mole
% of the total diamine is required.
Examples of dianhydrides that can be used to form positive-working
polyimide dielectric mixtures are pyromellitic dianhydride, 6F
dianhydride, biphenyl tetracarboxylic dianhydride, naphthalene
tetracarboxylic dianhydride, benzophenone tetracarboxylic
dianhydride, and bicyclodianhydride. A generalized structure of
dianhydrides is shown in FIG. 2.
The above polyamic acid can be mixed with a second polymer to
improve the developing latitude. Novolac in percentages of
approximately 10-20% improves the shape of the line profiles and
improves the reproducibility of the development rate. Other
additives which perform the same function are
dimethylaminobenzaldehyde, diisopropyl amine, trihexyl amine,
dibenzyl amine, N-(triethoxysilylpropyl) urea, p-dimethylamino
benzaldehyde, or isocyanatopropyltriethoxysilane. These additives
cause greater solubility difference between the exposed and
unexposed regions.
The base polyamic acid can be mixed with a polyurethane in
percentages lower than 5% to further slow the developing rate. The
use of a polymeric additive to slow development rate is expected to
be advantageous from the standpoint of dielectric properties
because the tendency to outgas to form pinholes is less. A second
advantage of the polymeric additive is that, even in small
percentages, it makes a significant contribution to the film
thickness because of its high molecular weight.
The base polyamic acid can be mixed with a cyclo aliphatic
diepoxide, examples of which are
3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate or the
Cyracure UVR crosslinking agents sold by Union Carbide, to slow
development rate. It is expected that the epoxy groups react with
the carboxylic acids during the bake step to retard solubility.
The polymers are synthesized by condensation of the diamines with
the dianhydrides in a solvent such as N-methylpyrrolidinone,
dimethylacetamide, dimethylformamide, or a mixture of the above
with a nonsolvent such as diglyme, 2-methoxyethylacetate, or other
ketones, ethers, or aromatic solvents. The formulations containing
solvent/nonsolvent mixtures have superior spin-coating
properties.
Besides the choice of monomer, the molecular weight of the polymer
is important to the resolution capability. If the molecular weight
is too high, incomplete development and scumming occurs. If the
molecular weight is too low, the polymer is too soluble in the
unexposed areas. As a result, triangular line profiles limit the
attainable resolution in low molecular weight polymer The molecular
weight can be limited by adjusting the mole ratio of the two types
of monomers (dianhydride and diamine). The actual molecular weight
range is dependent upon the desired film thickness.
For thicker photoresist films (greater than 5 microns), the
absorption of the base polymer at the exposure wavelength becomes
critical because of attenuation of the exposing radiation at lower
regions of the film. Table 1 shows polymers, absorbances, and
percentages of incident light transmitted I/I.sub.o .times.100) at
the bottom of a 10 micron film. Novolac, which is commonly used in
positive photoresist formulations, has an absorbance in the same
range as the absorbance of the BTDA-BAPS/4APS polymer described
here. For thicker film resists, the absorbance can be greatly
decreased by the substitution of BTDA with a monomer such as 5
(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,1-dicarboxylic
anhydride, which is sold by Dianippon as Epiclon B-4400.
Absorbances of the polymer of Epiclon B4400 with BAPS and 4APS are
shown in Table 1, and it can be seen that this polymer has a much
more preferable absorbance than the polymer containing BTDA.
TABLE 1 ______________________________________ Transmission of
exposure of selected polymers at 10 micron depth Absorbance of 10 u
film % transmitted at bottom at wavelength (nm) of 10 u film
Polymer 365 405 436 365 405 436
______________________________________ Novolac 1.03 0.48 0.39 9 32
41 BAPS.sup.1 2.15 0.45 0.21 1 35 61 EPI.sup.2 0.08 0.05 0.04 83 89
91 ______________________________________ .sup.1 BAPS denotes the
BTDABAPS/4APS polymer of Example 3. .sup.2 EPI denotes a polymer
identical to 1 above, except substituting an equimolar amount of
Epiclon B4400 (available from Dianippon Chemical) for the BTDA.
The photoactive sensitizer is the
1-oxo-2-diazo-naphthalene-5-sulfonic acid ester of
2,3,4-trihydroxybenzophenone Sensitizer from two manufacturers were
used: Fairmount #1006 and ICI #155. Both products have similar
performance.
An adhesion promoter such as a bifunctional silane which can couple
to the surface and also to the polymer, an example of which is
methacryloxypropyl trimethoxy silane, is spun onto the wafer before
the polyimide resist. The photosensitive polyimide compositions are
then spin-coated to th desired thickness. The films are heated to
evaporate solvent by baking on a hotplate at a temperature from
100.degree.-140.degree. C. for about 90 seconds. In place of the
hot plate, the films can be baked in a convection oven.
The photosensitive polyimide is then exposed to ultraviolet
radiation. Exposure can be either through a mask or by a directed
beam of radiation. Patterns are distinguished by developing in
aqueous basic developer solution Examples of developer are those
containing tetramethylammonium hydroxide (TMAOH), TMAOH with a
surfactant such as cetyl trimethyl ammonium bromide, aqueous KOH,
choline, or tetrabutylammonium hydroxide. For dielectric
applications, the metallic ion-containing solutions should be
avoided.
To cure the polyimides for dielectric applications, the films are
baked after exposure at several temperatures to remove solvent and
to imidize the polymer. The resistivity of BTDA-BAPS/4APS is
unaffected by the presence of sensitizer or Novolac, and for the
unexposed and undeveloped polyimide resist is in the range of
10.sup.16 ohm cm. Exposing the coating to developer prior to the
final bake is not detrimental to the dielectric properties, as
water that is added in the development step is removed in baking
stages. AC and DC dielectric properties are given in the
examples.
A primary application of the polyimide-based resist is to provide
dielectric isolation between multilevel metal structures on an
integrated circuit. In one application shown in FIG. 3, the
polyimide is used as a single-layer dielectric coating that would
not require a top layer of photoresist to be patterned. In a second
application, the insulating layer would be a sandwich structure of
polyimide and oxide shown in FIG. 4.
In the application shown in FIG. 4, the imagable polyimide would
take the place of photoresist which would be necessary to image the
metal or oxide layers. Following imaging, however, the polyimide
resist would not be stripped but would remain a part of the device.
When in combination with an oxide layer, the
polyimide-oxide-polyimide sandwich structure is expected to be less
susceptible to cracking due to mismatch of the thermal expansion
coefficients of the metal and oxide. The sandwich structure in FIG.
4 would have fewer failures due to pinholes because of the presence
of three separate insulating layers.
If silicon-containing polyimide solutions are used, the polyimide
can function as a dry etch mask over the top layer of a two-layer
resist structure. The lower layer serves either to level underlying
topography, or to provide dielectric insulation. In the latter
application, the polyimide resist has an advantage in that its
removal after etching is not required.
Variations of the polyimide-based resist or its processing can
produce materials suitable for thick-film coatings for hybrid
circuits, thick-film photoresists, passivation coatings, and
heat-resistant photoresists (for applications as a reactive ion
etch mask or an ion implantation mask).
The invention will be further understood by reference to
applicants' examples included herein.
EXAMPLE 1
This example reproduces Example 1 of EPO Patent Application No. 224
680.
A polyamic acid was prepared by mixing 20 g 4,4'-oxydianiline,
21.36 g pyromellitic dianhydride, 98 milliliters of
N-methylpyrrolidone, and 79 milliliters of 2-ethoxyethanol
(Cellosolve). A photoresist composition was prepared by mixing 20 g
of the above polyamic acid and 0.524 g of
2,3,4-trihydroxybenzophenone,l,2-naphthoquinone-(diazide-2)-5-sulfonate
(mixture of esters) containing a minimum of 75% triesters
(hereafter designated as 215THBP). The photoresist was diluted with
dimethylformamide in order to get coatings of 2 micrometers when
spun onto silicon dioxide wafers. To improve adhesion of the film
to the wafer, APX-Kl adhesion promoter (from Brewer Science, Inc.)
was precoated. The solvent was removed after spin coating by
prebaking the wafers at 95.degree. C. for 25 minutes The
photoresist was exposed for 90 seconds through a high resolution
test mask with a 200 watt medium pressure mercury arc lamp operated
at and incident intensity of 4,450 microwatts/cm.sup.2.
The wafers were developed in 0.23 N KOH for 1 to 60 seconds at
25.degree. C.
Both exposed and unexposed areas dissolved in the developer in a
few seconds. No patterns were observed during development.
EXAMPLE 2
This example substantially reproduces the examples in U.S. Pat. No.
4,093,461, except that a commercially available sensitizer is
used.
A photoresist composition was prepared by mixing 20 grams of the
above polyamic acid solution and 0.738 g of THBP sensitizer. This
photoresist was diluted with dimethylformamide in order to obtain
coatings of 1.2 micrometers when spun onto silicon wafers To
improve adhesion of the film to the wafer, APX-Kl adhesion promoter
was precoated. The solvent was removed after spin coating by
prebaking the wafers at 80.degree. C. for 1 hour. The photoresist
film was exposed through a high resolution test mask with a 200
watt medium pressure mercury arc lamp. The incident intensity used
was 4,450 microwatts/cm.sup.2 and the exposure time was 35
seconds.
Films were developed in diethylaminoethanol diluted with water at
1:15 v/v basis for 5 to 20 seconds at 25.degree. C. Both exposed
and unexposed areas dissolved in the developer in a few seconds. No
patterns were visible in the film during development.
EXAMPLE 3
A positive-working thermally stable photoresist composition was
prepared in the following way:
A polyamic acid was prepared using 3,3',4,4'-benzophenone
tetracarboxylic dianhydride (BTDA), 4-aminophenyl sulfone (4APS),
and bis-4-(4-aminophenoxy)phenyl sulfone, (BAPS). The molar ratio
between diamines 4APS/BAPS was 0.55/0.45, while the molar ratio
between dianhydride and total diamines was 0.8/1.0. This polyamic
acid was synthesized using a mixture of solvents
diglyme/dimethylformamide (60/40 by weight) with a solid content of
25%. The weight average molecular weight was 60,000 and the number
average molecular weight was 30,000.
A diazoquinone photosensitizer was incorporated in the mixture of
the polyamic acid-novolac solution in a concentration range between
10 to 20% by weight of the solids This solution was diluted with
diglyme/DMF (60/40 by weight) in order to form films of 1
micrometer thick when spin-coated between 3000 to 5000 rpm for 60
seconds and baked at 130 or 135.degree. C. for 2 minutes on a hot
plate.
The positive thermally stable photoresist solution prepared in this
way was applied to the surface of a silicon wafer which had been
coated previously with a solution of 0 075%
methacryloxypropyltrimethoxy-silane and 0025% of
N-[-3-(triethoxysilyl) propyl]-4,5-dihydroimidazole adhesion
promoter by means of a spin coating process at 5000 rpm for 30 sec
and baked at 115.degree. C. for 30 seconds. The photoresist-coated
wafer was partially imidized by a prebake of 130.degree. or
135.degree. C. for 2 minutes on a hot plate The photoresist film is
exposed through a pattern mask with a mercury lamp of a wavelength
of 365 nm with a dose of 150 mJ/cm.sup.2.
The exposed film was developed in a solution of MF-312 (a solution
containing 5% of tetramethyl ammonium hydroxide produced by Shipley
Company, Inc.) diluted with deionized water 1/1 by volume for 10 to
15 seconds to resolve 5 micrometer line and space geometries.
EXAMPLE 4
A positive thermally stable photoresist formulation was prepared
using the same base polyamic as in Example 1 mixed with 10% by
weight of solids of Cyracure UVR 6100, a cylic aliphatic diepoxide
manufactured by Union Carbide. The resist was spun, baked, and
patterned as in Example 1. The exposed film could be developed in
A-Z-327 MIF developer (a solution containing 5% of tetramethyl
ammonium hydroxide and a surfactant) for 5 seconds to resolve 1.7
micrometer line and space geometries with good edge definition. An
alternate developer was MF-312 diluted with deionized H2O (1/1 by
volume) mixed with 0.05% or 0.1% by weight of Cetyl trimethyl
ammonium bromide. Sensitivity was 70 mJ/cm.sup.2. The volume
resistivity of the fully cured photosensitive polyimide was
1.times.10.sup.16 ohm cm at an electric field of 1.times.10.sup.5
V/cm. The dielectric strength was greater than 5.times.10.sup.5
volt/cm. The dissipation factor was 0.003 (1 KHz) and the
dielectric constant was 2.7.
EXAMPLE 5
A positive thermally stable photoresist formulation was prepared
using the same base polyamic acid as above mixed with a novolac
resin (acid-catalyzed condensation product of o-cresol
2-t-butylphenol formaldehyde oxalic acid dihydride). The
concentration of Novolac incorporated was about 10% by weight of
the solids of the polyamic acid. The resist was spun, baked, and
patterned as in Example 1. Two micrometer line and space geometries
in a one micrometer film were resolved after 10 seconds of
development. The volume resistivity of the fully cured
photosensitive polyimide was 3.times.10.sup.16 ohm cm at an
electric field of 1.times.10.sup.5 V/cm The dielectric strength was
greater than 5.times.10.sup.5 volt/cm. The dissipation factor was
0.003 (1 KHz) and the dielectric constant was 2,7. The film had a
20% thickness loss following baking at 350.degree. C. for 30
minutes under nitrogen atmosphere.
EXAMPLE 6
A positive thermally stable photoresist composition was prepared in
the following way:
Polyamic acid was synthesized as in Example 1 and was mixed with
0.3-2% of one of the following compounds; diisopropyl amine,
trihexyl amine, dibenzyl amine, N-(triethoxysilylpropyl)-urea,
p-dimethylamino benzaldehyde, or isocyanatopropyl
triethoxysilane.
After addition of one of these components, 215THBP sensitizer was
added to the solution in a concentration of 15 to 20% of solids.
The films were spun, exposed, and developed as in Example 1. The
resulting patters had resolution of 2 micrometer lines and spaces
with good edge profiles.
EXAMPLE 7
The polyamic acid of Example 1 was mixed with 1% of a 50% solution
of a polyurethane in N-methylpyrrolidone. The polyurethane was
synthesized by condensing ethylene glycol with methylene
diisocyanate. The polymer was patterned as before to produce
positive-working patterns.
EXAMPLE 8
A polyamic acid was prepared using
3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4-aminophenyl
sulfone (4APS), and bis (4-[4amino phenoxy] phenyl sulfone) (BAPS).
The molar ratio between amines 4APS/BAPS was 0.55/0.45, and the
molar ratio between dianhydride and total diamines was 1/1. The
solvent used was a mixture of giglyme and dimethylformamide in a
ratio of 60/40 by weight. The solids content was about 20%.
A photoresist was prepared by mixing the above polyamic acid and
215THBP sensitizer in a concentration of about 30% of the polyamic
acid solids. The coating process was the same described in Example
1. The prebaking temperature was also about 130.degree. to
135.degree. C. for 2 minutes on a hot plate. The exposure dose was
100 mJ/cm.sup.2, and the developer used was a solution of MF312
diluted with water 1/1 by volume. Five micrometer line and space
geometries were obtained in 1 micrometer film thickness after 10
minutes of development.
A lower molecular weight modification of the above polyamic acid
was synthesized by reducing the mole ratio to 0.8/1. This polyamic
acid was mixed with 20% of 215THBP sensitizer (by weight of the
polyamic acid), spin coated onto silicon wafers, and baked at the
same conditions described in example 1. The film was then exposed
at 140 mJ/cm.sup.2, and developed with the same developer solution
as in Example 1 for 8 minutes. The line and space geometries
obtained were up to 5 micrometers in 1 micrometer thick film.
EXAMPLE 9
A thick-film positive-working polyimide resist was prepared in the
following manner:
A polyamic acid was synthesized as in Example 1 except that the
percent solids was 30%. Sensitizer (20% by weight of polyamic acid)
and Novolac (10% by weight of polyamic acid) were added. The resist
was spun and patterned as in example 1. In a six micron coating, 10
micron lines were obtained.
It will be appreciated by those skilled in the art that variations
in the invention disclosed herein may be made without departing
from the spirit of the invention. The invention is not to be
limited by the specific embodiments disclosed herein but only by
the scope of the claims appended hereto.
* * * * *